There are a couple of architectures on Gentoo that have a 32-bit userland, but need a 64-bit kernel compiler. kgcc64 is an attempt to provide one package for all of those architectures rather than the gcc-<arch> packages that exist now. The way we migrated over to this caused the issue you are seeing.

Yes, it's ok to unmerge gcc-sparc64 and merge kgcc64 instead.
As weeve said some architectures had the gcc-"arch"64 compiler and it was decided that we'd go into a unified kgcc64 so as to maintain the compiler easily and avoid duplicate work._________________Gustavo Zacarias
Gentoo/SPARC monkey